Process and apparatus for the extraction and regeneration of acids from spent acids

ABSTRACT

Process and apparatus for regenerating spent acid liquor includes a primary roasting furnace for evaporating a substantial portion of the liquid from the spent acid to produce acid vapors and partially roasted metal salts. The partially roasted metal salts are transferred to a secondary roasting chamber where the acids adhering to the surface of the metal salts is vaporized and the metal salts are oxidized. The acid vapors from the primary roasting furnace are then transferred to an absorption column to regenerate the acid. The primary roasting furnace is operated at a different temperature from the secondary roasting chamber and has different retention times for the metal salts. The secondary roasting chamber includes a raking device to mix and convey the metal salts during the secondary roasting step to produce a uniformly roasted metal oxide.

FIELD OF THE INVENTION

[0001] The present invention is directed to a process and apparatus forthe regeneration of acids by pyrolysis of a spent acid liquor solution.More particularly, the invention is directed to a process and apparatusfor the roasting of spent acids in a primary roasting furnace followedby a secondary roasting chamber.

BACKGROUND OF THE INVENTION

[0002] Pickling acids, such as hydrochloric acid and nitric acid, areused in the treatment of iron and other metals. Typically, the spentacids are regenerated for reuse in various processes to minimize wasteand problems associated with disposal.

[0003] Several acid regeneration processes pass the spent acid through afurnace to pyrolyze or roast the various components and to produce acidvapors. The vapors are passed through an absorption columncounter-currently to an absorption liquid which is typically water. Theregenerated acid is then withdrawn through the bottom of the column. Thepyrolysis exhaust gases are withdrawn from the head space of theabsorption column by an external exhaust fan which is supplied with awater spray to separate particulates remaining in the exhaust gas steambefore discharging through a chimney.

[0004] Pickling acids of hydrochloric acid used to treat iron or steelproduce numerous metal chlorides in the spent acid liquor. Many metalchlorides can be thermally converted into metal oxides and hydrogenchloride. The metal oxides then can be separated from the gaseoushydrogen chloride which can be recovered as hydrochloric acid forregenerating the pickling acids.

[0005] Typical processes of regenerating spent acids spray the aqueousacid solution into a heated reactor. The heated reactor heats the acidsolution to vaporize the aqueous phase and acid compounds and produceparticulates of metal salts that are contained in the spent acids asimpurities and waste byproducts. The droplets of the spent acid solutionand the particulates of the metal salts fall downwardly through thereactor where the particulates are removed. This process results in varyshort retention time in the reactor, usually only a few seconds, whichoften produces metal oxide particulates having acid residues on thesurface of the metal oxide particulates. The acid residues contaminatethe metal oxides, thereby requiring further processing before the metaloxides can be used. The reactor temperature can be increased tocompletely separate the acid residues from the metal oxide particulates.Increasing the reactor temperature often overheats and decomposes aportion of the metal oxide particulates. Furthermore, the acid liquor isusually injected into the middle of the reactor so that the droplets inthe middle are insufficiently heated, while the droplets on the outerareas of the reactor are overheated and partially decompose. Inaddition, increasing the reactor temperature results in a time lagbefore the desired roasting conditions are attained which can result inlarge amounts of acid residues being discharged with the metal oxidesparticulates. Other examples of processes for regenerating acids aredisclosed in U.S. Pat. No. 4,049,788 and U.S. Pat. No. 5,149,515.

[0006] Accordingly, a continuing need exists in the industry for animproved process for the roasting of spent acid liquors.

SUMMARY OF THE INVENTION

[0007] The present invention is directed to a process and apparatus forregenerating spent acids, and particularly pickling acids. Moreparticularly, the invention relates to a process and apparatus for thespray roasting of spent acid liquor using a roasting furnace having asecondary roasting chamber.

[0008] Accordingly, a primary object of the invention is to provide aprocess and apparatus for the complete and uniform roasting of a spentacid liquor and recovering the acid vapors.

[0009] Another object of the invention is to provide a process andapparatus for heating a spent acid liquor in a roasting furnace toseparate a substantial portion of the acid vapors from the metal saltsand transferring the metal salts to a secondary roasting chamber forfurther roasting of the metal salts.

[0010] A further object of the invention is to provide a process andapparatus for the spray roasting of spent pickling acids to produce ametal oxide of substantially uniform composition.

[0011] Still another object of the invention is to provide a process andapparatus for roasting a spent acid liquor to evaporate the water fromthe liquor and a secondary roasting chamber to heat and oxidize themetal salts.

[0012] The objects of the invention are basically attained by providinga process for regenerating spent acid liquor, comprising the steps of:feeding a spent acid liquor containing a metal salt into a primaryroasting furnace and roasting the liquor to produce an acid vapor and apartially roasted metal salt; discharging the partially roasted metalsalt from the roasting furnace to a secondary roasting chamber andcompletely roasting the metal salts to produce a metal oxidesubstantially free of acid residues; discharging the acid vapors fromthe primary roasting furnace and feeding to an absorption column; andfeeding an absorption liquid to the absorption column to contact theacid vapors and produce a regenerated acid solution.

[0013] The objects of the invention are further attained by providing anapparatus for regenerating spent acid liquor, comprising: a primaryroasting furnace having a spent acid liquor inlet, a heating device forheating spent acid liquor to evaporate a liquid portion and produce acidvapors and to produce a partially roasted metal salt, a first outlet fordischarging acid vapors, and a second outlet for discharging partiallyroasted metal salt; a secondary roasting chamber having an inlet forreceiving the partially roasted metal salt, a heating device for heatingthe secondary roasting chamber to a secondary roasting temperature tooxidize the metal salt substantially to a metal oxide and separate acidresidues from the metal salt and an outlet for the acid vapors; and anabsorption column for receiving acid vapors from the primary roastingfurnace, and having an absorption liquid inlet for contacting the acidvapors and forming regenerated acid.

[0014] These and other advantages and salient features of the inventionwill become apparent from the annexed drawings and the detaileddescription of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Referring to the drawings which form a part of the originaldisclosure in which:

[0016]FIG. 1 is a side view in cross-section of the roasting apparatusin a preferred embodiment of the invention;

[0017]FIG. 2 is a top view in cross-section of the apparatus of FIG. 1taken along line II-II of FIG. 1;

[0018]FIG. 3 is a top view in cross-section of the apparatus of FIG. 1taken along line III-III of FIG. 1; and

[0019]FIG. 4 is a schematic diagram of an acid regeneration apparatus.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention is directed to a process and apparatus forroasting spent acid liquor in the regeneration of acid. Moreparticularly, the invention is directed to a process and apparatus forroasting a spent acid liquor to separate acid vapors and produce a metaloxide having a substantially uniform composition.

[0021] The process and apparatus are suitable for large scale acidregeneration plants. The present invention primarily relates to a sprayroasting furnace for roasting the spent acids and recovering the acidvapors. In further embodiments, the spent acid liquor can be heated in afluidized bed reactor. The acid vapors are recovered from the furnaceand carried to further processing devices such as scrubbers andabsorbers to regenerate the acids. Suitable processing devices forregenerating acids from acid vapors are those generally known in the artof regenerating waste acids. An example of an apparatus for regeneratingacid solutions from acid vapors is disclosed in commonly owned U.S. Pat.No. 5,149,515 to Karner which is hereby incorporated by reference in itsentirety.

[0022] The process and spray roasting apparatus of the invention isparticularly suitable for spent acids used in the treatment of variousmetals. Examples of suitable acids include hydrochloric acid, nitricacid, hydrofluoric acid, sulfuric acid, and mixtures thereof. Suitablespent acids can include spent pickling acids used in the treatment ofiron, chromium, nickel, titanium, zirconium, aluminum, and the like. Thespent pickling acids typically contain large amounts of metal saltswhich must be separated or decomposed during the roasting process toregenerate the spent acid efficiently. Typically, the acids are roastedto vaporize the acids and oxidize the metal salts.

[0023] Referring to FIG. 1, the spray roasting apparatus 10 in apreferred embodiment of the invention includes a primary roasting zone12 and a secondary roasting zone 14. The spray roasting apparatus 10includes a substantially circular side wall 16 and a frustoconicalbottom wall 18 converging toward the center axis. A discharge opening 20is provided in the bottom wall 18 and positioned substantiallyconcentric to a vertical axis of the apparatus 10. An annular collar 19extends downwardly from the bottom wall 18 and surrounds opening 20. Thespray roasting apparatus 10 further includes a top wall 22 having anopening 24. In the embodiment illustrated, a pipe 26 extends throughopening 24 into the top end of the primary roasting zone 12. A nozzle 28is attached to the end of pipe 26 for spraying the spent acid solutioninto the primary roasting zone 12. A burner 30 is connected to a feedpipe 32 for feeding hot combustion gases into the primary combustionzone. An outer support structure 33 surrounds the outer wall 16 of theprimary roasting zone 12.

[0024] A housing 34 is coupled to the lower end of the spray roastingapparatus 10. Housing 34 includes a substantially cylindrical side wall36 and a bottom wall 38. Generally, bottom wall 38 is a flat wallsubstantially perpendicular to side wall 36. In the embodimentillustrated, side wall 36 is coupled to the conical bottom wall 18 ofthe apparatus 10. Housing 34 encloses and defines the second roastingzone 14 and is positioned directly below the primary roasting zone 12.

[0025] As shown in FIG. 1, discharge opening 20 of the conical shapedbottom wall 18 has a substantially annular shape at the apex of theconical bottom wall 18. A cone 40 is positioned in the opening 20 torestrict the size of the opening and restrict the size of particlesflowing from the primary roasting zone 12 into the secondary roastingzone 14 as discussed hereinafter in greater detail. Cone 40 includes asolid tip 42 having a collar 44 for coupling with a shaft 46. Afrustoconical shaped wall 48 is coupled to the tip 42 and terminates ata substantially cylindrical shaped side wall 50.

[0026] Cylindrical side wall 50 is positioned in the discharge opening20 and is dimensioned to provide a small gap 21 between the side wall 50and the annular collar 19 and discharge opening 20. In embodiments ofthe invention, cylindrical side wall 50 includes a plurality of ridgesor ribs 52 to assist in breaking the particles into smaller pieces asthey pass from the primary roasting zone 12 through the dischargeopening 20 into the secondary roasting zone 14. Shaft 46 is coupled to amotor 54 for rotating the cone 40 which feeds the particulate materialsfrom the primary roasting zone 12 into the secondary roasting zone 14 ata substantially uniform and constant rate. The size of the gap 21between opening 20 in the bottom wall 18 and the side wall 50 of cone 40is selected to define the particle size of the metal oxides passing intothe secondary roasting zone 14. Generally, the particles entering thesecondary roasting zone 14 are about 1 to about 300 microns. Inpreferred embodiments, the gap 21 is sufficiently small to effectivelyseparate the primary roasting zone 12 from the secondary roasting zone14.

[0027] As shown in FIGS. 1 and 2, a heating device 54 is included in thesecondary roasting zone 14. In the embodiment illustrated, heatingdevice 54 is a hollow annular shaped tube 56 having a substantiallysquare cross-section. An inlet pipe 58 is coupled to the heating tube 56for feeding heated fluid into the tube 56. A burner 60 is positioned inthe inlet pipe 58 to burn fuel and combustion air for heating the tube56. An exhaust outlet 62 is also coupled to the tube 56 for thedischarge of the exhaust gases as shown in FIG. 2. In the embodimentillustrated, the combustion gases are fed into the inlet pipe 58 andcirculate through the annular shaped heating tube 56 and are dischargedthrough the exhaust outlet 62. Generally, the heating tube is concentricwith the discharge opening 20. In further embodiments, heating device 54can be a radiant tube which can be heated indirectly by suitableprocesses, such as, for example, superheated steam.

[0028] A mixing and conveying device 64 is provided in the bottom end ofthe secondary roasting zone 14 adjacent bottom wall 38. In preferredembodiments of the invention, the mixing device 64 is a rake, such as arabble rake, having a support member 66 and a plurality of paddles 68.Support member 66 has a length slightly less than the diameter of thesecondary roasting zone 14. A plurality of blades or paddles 68 arefixed to the support member 66 and positioned at an angle with respectto a longitudinal dimension of the support member 66. In the embodimentillustrated, the paddles 68 extend downwardly from the support member 66and are closely spaced to bottom wall 38. In alternative embodiments,the support member 66 can be positioned proximate bottom wall 38 withthe paddles 68 extending upwardly.

[0029] Support member 66 is also attached to the shaft 46. Shaft 46extends through the bottom wall 38 of the housing 34 and is coupled to adrive motor 70 for rotating the shaft 46 about its vertical axis.Rotation of the shaft simultaneously rotates the mixing device 64 andthe cone 40. Rotation of the mixing device 64 in the direction of arrows72 of FIG. 3 continuously mixes and conveys the particulate materialfrom the center axis of the housing 34 toward the outer wall 36. Anopening 74 in the side wall 36 of the housing 34 opens to a dischargechute 76. In this manner, rotation of the mixing device conveys theparticulate material along the side wall 36 while continuously mixingthe particulate material where it is discharged through the dischargechute 76.

[0030] In further embodiments of the invention, an inlet pipe 78 extendsthrough the side wall 36 of housing 34 for introducing gases or othermaterials into the secondary roasting zone 14. In embodiments of theinvention, a reducing atmosphere can be introduced into the secondaryroasting zone to react with the metal salts in the roasting zone.Examples of suitable reducing gases include hydrogen, water vapor,methane or carbon monoxide, and mixtures thereof. A reducing atmospherecan be maintained in the secondary roasting zone to control the extentof oxidation of the metal salts while driving off the acid residueswhich may be adhered to the particles.

[0031] In the process of the invention, waste acid, such as spentpickling acid, is fed through the pipe 26 and nozzle 28 and sprayed intothe primary roasting zone 12. Hot combustion gases from burner 30 arefed through pipe 32 into the primary roasting zone for heating androasting the spent acid. The droplets of the spent acid fall downwardlythrough the primary roasting zone 12 where a substantial portion of thewater and acid components vaporize. The water vapor and acid vapors arewithdrawn through opening 24 and directed to a suitable absorptioncolumn where an absorption liquid is contacted with the vapors toproduce a regenerated acid. The metal salts in the spent acid are atleast partially oxidized in the primary roasting zone 12 as they fallthrough the hot combustion gases. The extent of oxidation of the metalsalts is dependent on the retention time of the metal salts as they fallthrough the primary roasting zone and the temperature of the combustiongases. The retention time of the droplets and metal salt particles istypically only a few seconds before they reach the bottom of the primaryroasting zone. The selected temperature of the primary roasting zone isdependent on the composition of the spent acid and the desired extent ofoxidation of the metal salts. In preferred embodiments of the invention,the primary roasting zone is heated to a temperature of about 500° toabout 600° C.

[0032] The particulates formed in the primary roasting zone 12 which areprimarily composed of the metal salts fall downward along the conicalbottom wall 18 toward the outlet 20 and the cone 40. The motor 70 andshaft 46 rotate the cone about its axis which break up the oversizedparticulates into sufficiently small size such that they can passthrough the gap 21 between the cylindrical side wall 50 of cone 40 andthe edge of the opening 20 of the conical bottom wall 18. A plurality ofribs 52 are provided on the outer face of side wall 50 of cone 40 toassist in reducing the particle size of the metal salts. The metal saltsfall downwardly through the gap 21 into the secondary roasting zone 14and rest on the bottom wall 38 of housing 34.

[0033] A fuel and air mixture is fed to the burner 60 which heats theheating tube 56 to heat the secondary roasting zone to a desiredtemperature. Mixing device 64 being connected to the shaft 46 rotatessimultaneously with the rotation of cone 40 to continuously mix theparticles in the secondary roasting zone 14. Continuous mixing of theparticles by the mixing device 64 continuously exposes fresh surfaces ofthe particulates to ensure proper and uniform roasting and a uniformcomposition of the resulting metal oxides. As shown in FIG. 3, rotationof the mixing device 64 conveys the particulates toward the outer wall46 of the housing 34 toward the discharge outlet 76. Continuous rotationof the mixing device provides substantially uniform mixing andsubstantially uniform retention times for the particulates in thesecondary roasting zone.

[0034] The temperature of the secondary roasting zone can be selecteddepending on the nature of the metal salts and the desired extent ofroasting and is typically different from the temperature in the primaryroasting chamber. In one embodiment of the invention, the secondaryroasting zone is carried out at a temperature of approximately 400-500°C. The secondary roasting temperature in this range is particularlysuitable for producing oxides having high specific surface areas andalso for producing gamma iron. In further embodiments of the invention,the secondary roasting zone can be carried out at a temperature of about800-1000° C. The secondary roasting at the higher temperatures isparticularly suitable for roasting metal chlorides to produce metaloxides that are substantially free of acid residues. In still furtherembodiments, the reducing atmosphere can be fed into the secondaryroasting zone through the inlet pipe 78. Examples of reducing gasesinclude hydrogen, water vapor, methane, carbon monoxide, and mixturesthereof. The operating temperature of the secondary roasting zone 14 isindependent of the operating temperature of the primary roasting zone12. Typically, the secondary roasting zone is operated at a temperatureto drive off the remaining acid residues of the particles and completelyoxidize the metal salts without decomposing the oxides.

[0035] The process and apparatus of the invention are particularlysuitable for vaporizing the acid components of spent acid that areconveyed to a regeneration column and for producing metal oxides havinga substantially uniform composition. In embodiments of the invention,the primary roasting zone is operated at a lower temperature thanconventional roasting zones to prevent overheating and decomposition ofthe metal oxides formed in the primary roasting zone. The operatingtemperatures in the primary roasting zone and the retention time of thespent acid is sufficient to vaporize substantially all of the water andacid components from the spent acid liquor. The particulate metal saltsformed in the primary roasting zone are at least partially oxidized andtypically contain some acid residues adhering to the surfaces of theparticles. The retention time of the spent acid in the primary roastingzone is generally a few seconds.

[0036] The partially oxidized metal salts are then passed into thesecondary roasting zone where the particles are heated at a temperatureindependent of the operating temperature of the primary roasting zone.The continuous mixing and conveying of the particulates in the secondaryroasting zone ensure that the metal salts are uniformly heated to driveoff the remaining acid residues which are then discharged to theatmosphere or further processing facility. The speed of rotation of theshaft 46 and the mixing device 64 determine the retention time of themetal salts in the secondary roasting zone and is independent of theretention time of the spent acids in the primary roasting zone. Theretention time of the metal salts in the secondary roasting zone istypically longer than the retention time in the primary roasting zoneand can be about 3-5 minutes depending on the nature of the metal saltsbeing roasted. The actual retention time in the secondary roasting zoneis generally several minutes. In embodiments, the metal salts beingheated in the secondary roasting zone can have a retention time of 10 toabout 20 minutes to ensure complete oxidation of the metal salts andcomplete removal of the acid residues.

[0037]FIG. 4 is a schematic diagram of the acid regeneration assembly ofthe invention. The assembly includes the spray roasting apparatus 10having the inlet pipe 26 and spray nozzle 28. The combustion gases arefed through pipe 32 to pyrolyze the spent acids. Acid vapors are removedthrough line 82 to a dust separator 84. The gases then enter at leastone and generally two absorption towers 86 and 88 having an absorptionliquid therein. The acid in the bottom of towers 86 and 88 are withdrawnby pumps 90 and 92 and passed through heat exchangers 94 and 96 wherethe acid liquid is fed to the top portions of the towers 86 and 88. Theexhaust gases are then passed from tower 88 through scrubbers 98 and 100and eventually through a fan 102 for discharging the exhaust to theatmosphere.

[0038] While several embodiments have been shown to illustrate theinvention, it will be understood by those skilled in the art thatvarious modifications and changes can be made therein without departingfrom the scope of the invention as defined in the following claims.

What is claimed is:
 1. A process for regenerating spent acid liquor,comprising the steps of: feeding a spent acid liquor containing a metalsalt into a primary roasting furnace and roasting said liquor to producean acid vapor and a partially roasted metal salt; discharging saidpartially roasted metal salt from said roasting furnace to a secondaryroasting chamber and completely roasting said metal salts to produce ametal oxide substantially free of acid residues; discharging said acidvapors from said primary roasting furnace and feeding to an absorptioncolumn; and feeding an absorption liquid to said absorption column tocontact said acid vapors and produce a regenerated acid solution.
 2. Theprocess of claim 1, comprising roasting said spent acid liquor in saidprimary roasting furnace for a first period of time and roasting saidpartially roasted metal salts in said secondary roasting chamber for asecond period of time, wherein said second period of time is longer thansaid first period of time.
 3. The process of claim 1, comprisingroasting said spent acid liquor in said primary roasting furnace for atime sufficient to evaporate a substantial portion of liquid from saidliquor and roasting said partially roasted metal salt in said secondaryroasting chamber for sufficient time to evaporate any remaining liquidand substantially vaporize acids adhering to said metal salt.
 4. Theprocess of claim 1, comprising roasting said acid liquor in said primaryroasting furnace at a temperature of about 500° to about 600° C.
 5. Theprocess of claim 1, comprising roasting said metal salts in saidsecondary roasting chamber for about 3-5 minutes.
 6. The process ofclaim 1, wherein said primary roasting furnace is a spray roastingfurnace and said process comprises spraying said acid liquor into saidroasting furnace.
 7. The process of claim 1, comprising roasting saidmetal salt in said secondary roasting chamber at a temperature of about400° to about 500° C.
 8. The process of claim 1, comprising roastingsaid metal salt in said secondary roasting chamber at a temperature ofabout 800° to about 1100° C.
 9. The process of claim 1, comprisingroasting said metal salt in said secondary roasting chamber for about 10to about 20 minutes.
 10. The process of claim 1, comprising roastingsaid metal salt in said secondary roasting chamber in an atmosphereselected from the group consisting of hydrogen, water vapor, methane,carbon monoxide, and mixtures thereof.
 11. The process of claim 1,wherein said secondary roasting chamber includes a heating device forfurther roasting said partially oxidized metal salt.
 12. The process ofclaim 1, comprising continuously mixing and conveying said metal salt insaid secondary roasting chamber while roasting said metal salt in saidsecondary roasting chamber.
 13. The process of claim 1, wherein saidsecondary roasting chamber is positioned directly below said primaryroasting furnace and said primary roasting furnace includes an outlet ata lower end thereof and a rotating conical shaped member positioned insaid opening to define a gap therebetween and to separate said primaryroasting furnace from said secondary roasting chamber, said processcomprising passing said metal salts from said primary roasting furnacethrough said gap into said secondary roasting chamber.
 14. The processof claim 13, wherein said secondary roasting chamber includes arotatable mixing device and a discharge outlet, said process comprisingcontinuously rotating said mixing device to mix and convey said metalsalt toward said discharge outlet while roasting in said secondaryroasting chamber.
 15. An apparatus for regenerating spent acid liquor,comprising: a primary roasting furnace having a spent acid liquor inlet,a heating device for heating spent acid liquor to evaporate a liquidportion and produce acid vapors and to produce a partially roasted metalsalt, a first outlet for discharging acid vapors, and a second outletfor discharging partially roasted metal salt; a secondary roastingchamber having an inlet for receiving said partially roasted metal salt,a heating device for heating said secondary roasting chamber to asecondary roasting temperature to oxidize said metal salt substantiallyto a metal oxide and separate acid residues from said metal salt, and anoutlet for said acid vapors; and an absorption column for receiving acidvapors from said primary roasting furnace, and having an absorptionliquid inlet for contacting said acid vapors and forming regeneratedacid.
 16. The apparatus of claim 15, further comprising a member forpartially closing said second outlet in said primary roasting furnace todefine an evaporation zone in said primary roasting furnace separatefrom a secondary roasting zone in said secondary roasting chamber. 17.The apparatus of claim 15, wherein said heating device for saidsecondary roasting chamber includes a hollow substantially uniform ringand a burner device for passing hot combustion gases through said ring.18. The apparatus of claim 17, wherein said ring is substantiallyconcentric with said second outlet in said roasting furnace.
 19. Theapparatus of claim 15, wherein said primary roasting furnace is a sprayroaster.
 20. The apparatus of claim 15, wherein said secondary roastingchamber comprises a rake and a motor drive device for moving said raketo mix and convey metal salt in said secondary roasting chamber.
 21. Theapparatus of claim 20, wherein said rake comprises a support member witha longitudinal dimension and having a plurality of blades fixed to saidsupport member at an angle with respect to said longitudinal dimension,and said motor drive device is coupled to said support member forrotating said rake about a central axis with respect to said secondaryheating chamber to mix and convey metal salt from said center axistoward an outlet in said secondary roasting chamber.
 23. The apparatusof claim 20, wherein said rake is a rabble rake.
 24. The apparatus ofclaim 15, wherein said secondary roasting chamber comprises an inlet forfeeding reducing gas into said secondary roasting chamber.
 25. Theapparatus of claim 24, wherein said reducing gas is selected from thegroup consisting of hydrogen, water vapor, methane, carbon monoxide, andmixtures thereof.